Hydromodification is defined as changes in runoff characteristics and in-stream processes caused by altered land use. With regard to urbanization, when areas are rendered impervious with asphalt, concrete, and roofs, and runoff is conveyed directly to streams via the conventional storm drain system, natural storage in vegetation and infiltration into soil is reduced and overland flow increases. As a result, the magnitude and duration of flow rates entering receiving streams or other channels increases, which contributes to more erosive energy within the channel. Unless managed, hydromodification can cause impacts including channel erosion and results in biological impacts to stream systems as well as infrastructure.
The state of the practice for hydromodification management for new urban development is to mimic long-term pre-development site hydrology. The theory is that if the pre-development distribution of in-stream flows is maintained, then the baseline capacity to transport sediment, a proxy for the geomorphic condition, will be maintained as well. A popular method of mimicking the pre-development flow regime is via flow duration control. This can be done by routing post-development runoff through stormwater detention facilities, also termed structural Best Management Practices (BMPs), such that runoff is stored and slowly released to match pre-development flow rates and durations. Sizing such Hydromodification Management (HM) BMPs relies on continuous hydrologic simulation of both the pre- and post-development conditions and iterative design of the stormwater facility geometry and outlet configuration until flow duration control is achieved. As it turns out, storage requirements for flow duration control tend to be much larger than that for surface water treatment requirements, particularly when the stormwater facilities are small, distributed facilities with simple outlet structures, such as those designed for Low Impact Development (LID). These larger HM BMPs tend to: add cost to construction; be difficult to situate on a project site; and have long drawdown times resulting in vector control concerns.